This invention relates to a method of manufacturing an optical medium, and in particular, to a manufacturing method suitable for an optical fiber oscillator or an optical waveguide laser oscillator.
In a field of an optical communication or a laser process, it has been desirably required to develop a cheaper laser device having a higher output.
Conventionally, an optical fiber laser device has been known as a device satisfying this requirement.
In such an optical fiber device, a core diameter and index of refraction between a core and a clad are suitably selected. Thereby, a laser light beam of high quality can be relatively simply obtained.
Further, an interactive operation between laser active substance and the light beam can be enhanced by sealing the light beam with high density.
Moreover, an interactive operation length can be enlarged by lengthening the optical fiber. Thereby, the laser beam having the high quality can be spacially generated with high efficiency. Consequently, the laser light beam of high quality can be relatively cheaply obtained.
In this case, it is necessary to efficiently introduce an excitation light beam into a doping region (generally, referred to as a core portion) of a luminescent center (hereinafter, referred to as laser active substance), such as, a laser active ion, a color element of the optical fiber in order to realize further high-output and high-efficiency of the laser light beam.
Meanwhile, when the core diameter is generally set so as to satisfy a waveguide condition of a single mode, the diameter is restricted to several tens xcexcm or less. In consequence, it is generally difficult to efficiently introduce the excitation light beam for the small core diameter.
To solve such a problem, suggestion has been made about a double clads type fiber laser, for example, as disclosed in H. Zellmer, U. Willamowski, A. Tunnermann, and H. Welling, Optics Letters. Vol. 20, No.6, pp. 578-580, March, 1995.
In such a double clads type fiber laser, a first clad portion having lower index of refraction than the core portion is arranged around the core portion, and a second clad portion having further lower index of refraction is provided at an outside portion thereof.
Thereby, the excitation light beam introduced into the first clad portion propagates by total reflection due to a difference of index of refraction between the first clad portion and the second clad portion on the condition that the excitation light beam is sealed in the first clad portion.
During the propagation, the excitation light beam repeatedly passes the core portion to excite the laser active substance of the core portion.
In this event, the first clad portion has a large area of about hundred times in comparison with the core portion. Consequently, it is possible to further introduce many excitation light beams and to produce a high output.
In the double clads type fiber laser advantageously has high oscillation efficiency and a single and stable oscillation lateral mode.
Further, an output within the range between several watt to about 10 watt can be obtained by the use of a laser diode (hereinafter, abbreviated as a LD). In consequence, a further higher output can be realized as compared to the conventional core type fiber laser.
However, edge excitation is carried out from an edge or both edges of the fiber in such a double clads type fiber laser. Consequently, the number of the LDs for excitation can not be increased.
Namely, it is indispensable to achieve a high output of an introduced LD to produce a laser light beam having an high output in the double clads type fiber laser.
To solve this problem, suggestion has been made about a method for bundling a plurality of double clads type fiber lasers to achieve the high output. Although an average output can be increased with the bundled number, brightness is inevitably lowered. This reason will be explained below.
Namely, the core portions as luminescent points are widely dotted in a space because a larger clad portion (having a diameter of about 100 times) than the core is attached to the core portion. Thus, light-gathering performance can not be enhanced only by bundling a plurality of fiber lasers.
Therefore, another suggestion has been made about a laser device in which a fiber is repeatedly folded or wound in a disk-like region or a cylindrical region, and a plurality of LD light sources (beams) are inputted as excitation light beams from the periphery of the fiber laser having the disk-like structure or the cylindrical structure, as disclosed in Japanese Unexamined Patent Publication No. Hei. 10-135548, or Hei. 10-190097.
However, when these laser devices are manufactured, the fiber is dipped in resin and cured or hardened to keep these shapes after the fiber is formed to the disk-shape or the cylindrical shape.
In consequence, it is difficult to manufacture the fiber and to form to an arbitrary shape.
It is therefore an object of this invention to provide a method of manufacturing an optical medium which is readily capable of manufacturing it.
It is another object of this invention to provide a method of manufacturing an optical medium which is capable of improving a laser output by using a fiber laser which is superior in light-gathering performance and has a thermally stable output and a lateral mode.
According to a first aspect of this invention, an optical medium is manufactured by the following steps.
An extended optical conductor containing active substance is first formed to a predetermined shape by the use of resin by repeatedly folding or winding the optical conductor.
A laser light beam or an amplified light beam is outputted from an edge portion of the optical conductor by absorbing an excitation light beam incident from a side surface of the optical conductor into the active substance through the resin.
Thermoplastic resin is used as the resin. In this event, the thermoplastic resin transmits the excitation light beam.
The resin is heated up to a glass transition temperature or higher.
The optical conductor and the resin are bonded to each other so as to constitute a predetermined shape. Finally, the resin is cured.
In the first aspect, the fiber is repeatedly folded or wound in a region of a disk shape or a cylindrical shape. The excitation light beams can be incident through a plurality of LD light sources from the periphery of the fiber laser having the disk shape or the cylindrical shape. Thereby, the laser output can be increased.
The fibers must be bonded or welded and be integrated the desired shape in order to mold the fiber for the fiber laser to the disk shape or the cylindrical shape.
As the molding method, it may be readily assumed that the fiber containing glass as a main component is thermally welded.
However, when the fiber is formed by only the glass material in this method, thermal deformation temperature becomes high (several hundreds xc2x0 C. or higher). Consequently, the molding process is not easy.
Further, the fiber is often damaged in the molding process by an affect of scratches which may be made in the molding process. Therefore, the resin is generally coated so as to enhance the strength of the fiber.
The first aspect is characterized in that the thermoplastic optical resin is used as the resin material for coating the fiber. In this case, the thermoplastic optical resin can be readily and thermally molded.
Moreover, the thermoplastic optical resin can be coated onto the fiber surface by the in-line process in the step for making the fiber from the fiber preform as serving as the starting material. In consequence, the desired laser fiber can be readily produced.
In addition, thermal deformation generally occurs at a low temperature of about 200xc2x0 C. or lower in the thermoplastic resin. Consequently, the fiber laser having the disk shape or the cylindrical shape can be readily and thermally molded.
In this case, phenol resin or melamine resin is the thermosetting resin having a bridge point. When the heating temperature is gradually increased in the thermosetting resin, slight fluidization can be obtained. However, immediately after this, the thermosetting starts and proceeds. Consequently, it is difficult to thermally mold such resin.
In the meantime, resin deformation occurs when the thermoplastic resin is put into a high temperature state. When the resin is re-cooled, the resin is formed on the condition that the deformed structure is kept or maintained. As a result, the resin can be readily and thermally molded.
As the representative thermoplastic resin, acrylic rein, polycarbonate resin, polyethylene resin, polystyrene resin, vinyl chloride resin, and epoxy resin are exemplified. In this event, the resin may be polymer having a line-like structure, and can achieve the above-mentioned object of this invention.
In the thermoplastic resin, the temperature for starting the thermal deformation, namely, the glass transition temperature is approximately 100xc2x0 C. Consequently, the fiber coated by the thermoplastic resin is wound to the disk shape or the cylindrical shape, and is heated to 100xc2x0 C. to weld the fibers to each other.
Further, the coated resin is lowered to the desired viscosity as the state for thermally welding it. Thereby, the fibers can be more readily welded. Thereafter, the resin is cooled to the glass transition temperature or lower. Consequently, the fiber laser apparatus can generate the laser light beam having the high output.
In the second aspect of this invention, the optical conductor is preferably an optical fiber. When the optical conductor is formed by the optical fiber in which the core and the clad are integrated, the optical medium can has superior light-gathering performance, and the output and the lateral mode are thermally stable in the optical medium.
In the third aspect of this invention, the optical fiber has an outermost layer while the resin has first index of refraction. The outermost layer has second index of refraction. With this structure, the first index is equal to or substantially equal to the second index.
When the resin has the index of refraction equal to or substantially equal to the index of the refraction of the outermost layer of the optical fiber, the light beam can be readily introduced into the outermost layer of the optical fiber through the resin.
In the fourth aspect of this invention, the optical conductor is a core of an optical fiber. The resin has first index of refraction while the core has second index of refraction. In this condition, the first index is lower than the second index.
The optical conductor is composed of the core material constituting a part of the optical fiber, and the index of refraction of the resin is made lower than the index of refraction of the core to serve the clad material.
Consequently, the optical medium can be more readily manufactured, and further, the light beam can be sealed with high density.
In the fifth aspect of this invention, the optical conductor and the resin are formed to the predetermined shape by the thermal welding process or the thermal molding process.
The resin, which is readily molded, interposed between the conductors. Thereby, the general molding method, such as, the thermal welding and the thermal molding, can be adopted as the method for forming the optical medium to the desired shape.
In the sixth aspect of this invention, the resin contains bubbles. The bubbles are removed by reducing a pressure or by increasing a pressure during the thermal molding process.
The bubbles can be easily removed by reducing the pressure or by increasing the pressure during the thermal molding process.
In the seventh aspect of this invention, the optical conductor and the resin are formed to the predetermined shape by heating the resin in inert gas atmosphere.
When the resin is heated in the inert gas atmosphere, the resin does not occur a chemical change, and impurities are not mixed into the resin.
In the eighth aspect of this invention, the optical conductor and the resin are formed to the predetermined shape by previously coating the optical conductor with the resin, repeatedly folding or winding the optical conductor, and heating the optical conductor.
If the optical conductor is coated with resin in advance, it is unnecessary to coat the resin in the subsequent process. Consequently, the molding process for obtaining the optical medium having the desired shape becomes easy.
In the ninth aspect of this invention, the optical conductor is drawn into fiber. The liquid resin is applied to the optical conductor. The liquid resin is ultraviolet polymerized and/or thermal polymerized. In this event, the steps are carried out by an in-line process.
If the coating formation of the resin is carried out by the in-line process, the molding process for obtaining the optical medium having the desired shape becomes further easy.
In the tenth aspect of this invention, at least one portion of a surface of the resin is polished after curing the resin. In this case, at least one portion corresponds to the surface for reflecting the excitation light beam.
With such a structure, the surface itself of the resin becomes the surface for reflecting the excitation light beam. Alternatively, additional external resin for reflecting the excitation light beam may be coated with the surface of the resin.
More Specifically, at least one portion of the resin surface corresponds to an edge portion of the optical conductor for outputting or taking out the laser light beam, an edge portion of the optical conductor for reflecting the light beam, or an outer periphery surface of the optical conductor for introducing the excitation light beam.
Such a part is polished, and the surface for reflecting the excitation light beam or the external resin is removed. Thereby, the laser light beam is outputted or taken out in a simple manner. As a result, efficiency for reflecting the light beam at the edge portion and for introducing the excitation light beam can be enhanced.
In the eleventh aspect of this invention, the optical medium is manufactured by the use of the method according to the first aspect.
The excitation optical source is arranged such that the excitation light beam is incident from the side surface of the optical conductor and is absorbed into the active substance in order to excite the active substance. The laser beam is outputted from the edge of the optical conductor.
With this structure, much excitation light beams are incident from the side surface of the conductor of the optical medium. In consequence, the laser light beam having the high output can be obtained.
In the twelfth aspect of this invention, the optical medium is manufactured by the use of the method according to the first aspect.
The excitation optical source is arranged such that the excitation light beam is incident from the side surface of the optical conductor and is absorbed into the active substance in order to excite the active substance. The amplified light beam is outputted from the edge of the optical conductor.
With such a structure, much excitation light beams are incident from the side surface of the conductor of the optical medium. Consequently, much higher amplification degree can be obtained.